1. Field
The invention is in the field of equipment and methods for recovery of gold and/or silver from ores by leaching using a cyanide solution, adsorption of the gold and/or silver in solution onto activated carbon, and elution of the adsorbed gold and/or silver-cyanide complex for subsequent recovery.
2. State of the Art
In the mining and related industries, gold is typically recovered from ores and other such solid materials by cyanide leaching using an aqueous sodium cyanide leachant. The gold in the leach solution is in the form of a gold cyanide complex. The gold in the pregnant leach solution is adsorbed onto activated carbon using a Carbon-In-Pulp (CIP) process, a Carbon-In-Leach (CIL) process, or a Carbon-In-Column (CIC) process. Silver is recovered in the same manner as gold and thus silver and silver-cyanide complex can be substituted, respectively, for gold and gold-cyanide complex in this specification, claims, and abstract.
The CIP adsorption process involves a plurality of tanks, called stages, containing activated carbon which are positioned in a cascading fashion with the pregnant pulp, an aqueous cyanide solution with finely ground leached ore in suspension, entering and flowing through the first stage and serially down through the subsequent stages with the gold-cyanide complex therein being adsorbed by the carbon and exiting from the last stage depleted of most of the gold-cyanide complex. Activated carbon is fed into the last stage and moved serially through the stages against the flow of the solution through and out of the first stage for subsequent removal of the adsorbed gold-cyanide complex thereon. The activated carbon in each stage becomes loaded to pseudo-equilibrium which depends on the concentration of the gold-cyanide complex in each particular stage. The activated carbon in the first stage has the highest gold loading and is contacted with the pulp having the highest concentration of gold, the highest grade solution, while the activated carbon in the last stage having the lowest gold loading is contacted with the lowest grade pulp, with such activated carbon having the highest activity so as to more efficiently remove the last of the gold from the solution.
The CIL absorption process is a modification of the CIP process wherein the leaching of the gold from finely ground ore using the cyanide solution and the adsorption thereof by the carbon are done simultaneously in the same tanks.
The CIC absorption process is used in leap and other bulk ore leaching processes wherein the leaching of the gold from ore using cyanide solution results in a primarily clear pregnant solution and the adsorbtion thereof is done in the same manner as the CIP adsorption circuit.
The adsorbed gold-cyanide complex is stripped from the activated carbon typically using one of the three elevated temperature methods, the pressure ZADRA, the pressure AARL (Anglo American Research Laboratory), and the split pressure AARL. In all three processes, weak sodium cyanide and caustic solutions are heated to near the boiling point of the aqueous solution, then routed through a bed of gold-cyanide complex loaded activated carbon under a specified system pressure.
The pressure ZADRA method utilizes a pressure strip vessel, or strip column wherein strip solution having a concentration of approximately 1% sodium hydroxide (NaOH), and approximately 0.1% to 0.3% sodium cyanide (NaCN) is heated to a temperature of between about 290xc2x0 C. to 300xc2x0 C. at a pressure of between about 400 kPa to 500 kPa and is pumped in ascension through a vertical bed of loaded carbon residing in the strip column and discharges through a nozzle located at the top of the strip column. As the solution contacts the gold-cyanide loaded activated carbon, the combination of caustic and cyanide reagents and elevated temperature reverses the chemical equilibrium of the adsorbed gold-cyanide complex on the activated carbon resulting in the desorption of the gold-cyanide complex from the activated carbon into the strip solution. The gold is then recovered down stream of the strip column by electrowinning the pregnant strip solution, or by using the Merrill Crowe process. The pressure ZADRA system is conducted in a batch-by-batch process and requires approximately eight to sixteen hours to complete. Therefore, a back-to-back strip sequence requires eight-plus-eight to sixteen-plus-sixteen hours, or between about sixteen and thirty-two hours to complete.
The pressure AARL method utilizes a pressure strip vessel, or strip column wherein the loaded activated carbon is pretreated with an approximately 3% NaCN and 1% NaOH solution for about thirty minutes. The loaded activated carbon is then eluted with six to eight bed volumes of deionized water at a temperature of between about 110xc2x0 C. to 120xc2x0 C. and a pressure of between about 70 kPa to 100 kPa, which is pumped in ascension through a vertical bed of carbon residing in the strip column and discharges through a nozzle located at the top of the strip column. As with the pressure ZADRA method, the strip solution contacts the gold-cyanide complex loaded activated carbon, the combination of caustic and cyanide reagents and elevated temperature reverses the chemical equilibrium of the adsorbed gold-cyanide complex on the activated carbon resulting in the desorption of the gold-cyanide complex therefrom. The gold is later recovered by electrowinning or by using the Merrill Crowe process. Like the pressure ZADRA method, the pressure AARL method is conducted in a batch-by-batch process and requires approximately eight to sixteen hours to complete. Therefore, a back-to-back strip sequence requires eight-plus-eight to sixteen-plus-sixteen hours, or between about sixteen and thirty-two hours to complete.
The split pressure AARL method is similar to the pressure AARL with the exception of the final four bed volumes of deionized water strip solution saved in an intermediate solution tank and is then used as the first four bed volumes of strip solution of the next strip sequence.
A method for eluting a metal-cyanide complex comprising a gold-cyanide complex and/or silver cyanide complex from loaded activated carbon contained in respective first and second vessels using a strip solution, for example an aqueous solution of sodium hydroxide and sodium cyanide, and recovering the precious metal comprising respective gold and silver therefrom. The vessels are selectively connectable in series and each vessel individually to a device for recovering the precious metal, such as by the Merrill-Crowe method or by electrowinning (hereinafter recovery of such precious metal by any such method and device referred to as elecrowinning using an electrowinning device), forming respective continuous loops. A strip solution is selectively flowable through the respective loops using a pump connected therewith.
The method comprises a first step of flowing the strip solution in a first continuous loop as barren strip solution from the electrowinning device through the loaded carbon in the first strip column and back through the electrowinning device. The strip solution elutes metal-cyanide complex from the loaded activated carbon to produce a pregnant strip solution and a partially depleted loaded activated carbon in the first strip column. The electrowinning device removes the eluted precious metal from the pregnant strip solution.
A second step of the method comprises flowing the strip solution in a second continuous loop as barren strip solution from the electrowinning device, through the partially depleted loaded carbon in the first strip column and the loaded carbon in the second strip column, and back through the electrowinning device. The strip solution elutes the remaining metal-cyanide complex from the partially depleted loaded activated carbon in the first strip column to produce a pregnant strip solution and a mostly depleted barren activated carbon in the first strip column. The pregnant strip solution continues through the second strip column containing loaded activated carbon wherein the strip solution elutes metal-cyanide complex from the loaded activated carbon in the second strip column to produce a further pregnant strip solution and a partially depleted loaded activated carbon in the second strip column. The electrowinning device removes the eluted precious metal from the pregnant strip solution.
A third step of the method comprises flowing the strip solution in a third continuous loop as barren strip solution from the electrowinning device through the partially depleted loaded carbon in the second strip column and back through the electrowinning device. The strip solution elutes the remaining metal-cyanide complex from the partially depleted loaded activated carbon in the second strip column to produce a pregnant strip solution and a mostly depleted barren activated carbon. The electrowinning device removes the eluted precious metal from the pregnant strip solution.
The steps of flowing the strip solution in first, second, and third continuous loops are preferably conducted with the strip solution at a temperature elevated above ambient temperature and at a pressure elevated above ambient pressure.
The method preferably includes preheating the strip solution to an elevated temperature prior to initially flowing the strip solution in the first continuous loop, such as at the beginning of a work day. The preheating is done by flowing the strip solution in a fourth continuous loop from the electrowinning device through one or more heating devices and back through the electrowinning device. This is done until the strip solution is heated to a temperature sufficient for stripping the loaded activated carbon.
The method can be conducted in a batch process which includes an initial step of loading the first and second strip columns with loaded activated carbon. The first, second, and third steps are then conducted, following which a final step of removing depleted activated carbon from the first and second strip columns and loading a fresh batch of loaded activated carbon into each of the first and second strip columns is conducted. The first, second, third, and final steps can be repeated sequentially, together constituting batches.
The method can be conducted in a multiple consecutive batch process which includes the same initial step as the batch process of loading the first and second strip columns with loaded activated carbon. The first, second, and third step are repeated multiple times wherein an intermediate step of removing any depleted activated carbon from the respective first and second strip columns and loading a fresh batch of loaded activated carbon into the respective first and second strip columns is conducted prior to each of the first and third steps. This process permits unloading of depleted activated carbon and reloading with fresh loaded activated carbon of the respective first and second strip columns during the solo stripping of the other thereof.
The method can be conducted in a faster multiple consecutive batch process which includes the same initial step as the batch process of loading the first and second strip columns with loaded activated carbon. The second step is the same as for the previous processes. An alternative third step is utilized which comprises flowing the strip solution in an alternative third continuous loop as barren strip solution from the electrowinning device, through the partially depleted loaded carbon in the second strip column and the loaded carbon in the first strip column, and back through the electrowinning device. The strip solution elutes the remaining metal-cyanide complex from the mostly depleted barren activated carbon in the first strip column to produce a pregnant strip solution and a partially depleted loaded activated carbon in the first strip column. The pregnant strip solution continues through the second strip column containing loaded activated carbon wherein the strip solution elutes metal-cyanide complex from the loaded activated carbon in the second strip column to produce a further pregnant strip solution and a partially depleted loaded activated carbon in the second strip column. The electrowinning device removes the eluted precious metal from the pregnant strip solution. The second and alternative third steps are repeated multiple times wherein an intermediate step of removing the depleted activated carbon from the respective first and second strip columns and loading a fresh batch of loaded activated carbon into the respective first and second strip columns is conducted prior to each of the first and the alternative third steps. This process permits quicker cycle times since the second step and the alternative third step are both conducted with the first and second strip columns simultaneously, with the depleted activated carbon being replaced by fresh activated carbon in both strip columns between the steps.
The apparatus is for practicing the method of the invention by eluting a metal-cyanide complex comprising a gold-cyanide complex and/or silver cyanide complex from loaded activated carbon using a strip solution. The apparatus is further for recovering the precious metal comprising respective gold and silver from a pregnant strip solution formed by the eluting the loaded activated carbon using the strip solution to form depleted activated carbon.
The apparatus includes respective first and second strip columns fillable with the loaded activated carbon. The strip columns include respective fluid inlets and outlets for the strip solution to enter and exit so as to pass through the loaded activated carbon. The strip columns are unloadable of stripped activated carbon. The apparatus further includes an electrowinning device for removing the precious metal from the pregnant strip solution. The electrowinning device includes a fluid inlet for admitting the pregnant strip solution and a fluid outlet for exiting barren strip solution. A supply pipe assembly permits selective fluid connection of each of the strip columns to the electrowinning device to comprise respective continuous fluid conveying first and second loops. A crossover pipe assembly permits selective fluid connection of the outlet of one of the strip columns with the inlet of the other of the strip columns to comprise a third continuous fluid conveying loop. A pump operationally associated with the supply pipe assembly conveys the strip solution through the first, second, and third loops. The strip solution can be selectively pumped in the first loop through the first strip column, the supply pipe assembly, and the electrowinning device. The strip solution can alternatively be selectively pumped in the second loop through the first strip column then through the second strip column, the crossover pipe assembly, the supply pipe assembly, and the electrowinning device. The strip solution can alternatively be selectively pumped in the third loop through the second strip column, the supply pipe assembly, and the electrowinning device so as to more efficiently use the dynamics of the strip solution used therewith.
Preferably the columns and loops are sealable such that pressure above ambient can be selectively maintained therein using the pump to permit higher operating temperatures.
Preferably the crossover pipe assembly comprises a fluid crossover pipe and a crossover valve. The crossover pipe includes respective fluid inlet and outlet ends, the inlet end thereof being fluidly connected to the fluid outlet of the first strip column, and the outlet end thereof being fluidly connected to the fluid inlet of the second strip column. The crossover valve is disposed along the length of the crossover pipe for regulating the flow of strip solution therethrough.
Preferably the crossover pipe assembly further comprises a second fluid crossover pipe and crossover valve. The second crossover pipe includes respective fluid inlet and outlet ends, the inlet end thereof being fluidly connected to the fluid outlet of the second strip column, and the outlet end thereof being fluidly connected to the fluid inlet of the first strip column. The second crossover valve is disposed along the length of the second crossover pipe for regulating the flow of strip solution therethrough.
Preferably a heating device is connected to the supply pipe assembly between the fluid outlet of the electrowinning device and the fluid inlets of the first and second strip columns. The heating device heats the flow of strip solution prior to entering the first and second strip columns.
Preferably a cooling device is connected to the supply pipe assembly between the fluid outlets of the first and second strip columns and the fluid inlet of the electrowinning device. The cooling device cools the flow of strip solution prior to entering the electrowinning device.
Preferably the supply pipe assembly further comprises a bypass pipe and a bypass valve. The bypass pipe includes respective fluid inlet and outlet ends, the fluid inlet end thereof being fluidly connected intermediate the fluid inlets of the strip columns and the heating device, and the fluid outlet end thereof being fluidly connected ahead of the inlet of the electrowinning device. The bypass valve is disposed along the length of the bypass pipe for regulating the flow of strip solution therethrough. The bypass valve permits selective bypass of the strip solution exiting the heating device to the electrowinning device and back through the heating device in a continuous loop for preheating the strip solution prior to entering the strip columns.